Do upgraded LAN cables/purifiers/switch box make a difference. What is recommended, if so?

Before spending big bucks on a switch and cables (there really is no such thing as better cables (unless you mean physically built better), cat 6 would be more than sufficient and just make sure you have a brand name Gigabit switch.

DACs/Streamers may have audible differences but primarily on the analog side of the DAC assuming it’s designed by a competent engineer and that’s probably why people hear a difference between them.

If there is something you’re dissatisfied with your current setup, unless you bollixed up your network or you have insufficient bandwidth from your ISP (which I doubt), the network is almost never the problem.  I’d look elsewhere.

+1 for the Etherregen and the new v2 is likely to come out in Q1/26. Get it with Ca6 BlueJeans ethernet cable. 

Based on my recent experiences. In the order from discernable to impressive: ethernet cables https://www.amazon.com/dp/B07VVFB8VX?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1, linear power supply for modem and router, Network Acoustic Eno2 https://www.networkacoustics.com/shop/eno2-ethernet-filter/?srsltid=AfmBOoqLAY5mQsun5bBZ39LfnaXK-_xIEc-7AS2Ef9m3xdtjS_ocD-5q.

Into Aurender N100 and Denafrips VenusII. 

No, they make no objective difference in the sound from a stereo.

I bought a couple of LAN silencer and purifier products from SilentPower (a division of iFi) and it made a difference in my system.  Everything was much quieter, and to my ear there was more definition and clarity in the music from not only my internal NAS/streamer but also from the Internet.
 

The LAN iPurifier Pro is the primary unit and I placed it between my router and my first switch (which is a standard 8-port switch and nothing that is geared toward audiophile use), which then goes off to my computer and the NAS.  I also bought a couple of the lesser LAN iSilencer products and plugged one of those directly into the LAN feed to the NAS and also placed one on my streamer.  I never once considered sending these back they performed as advertised.

https://www.silentpower.tech/products/lan-ipurifier-pro